An organic EL device is a spontaneous light emitting device which utilizes such a principle that a fluorescent substance emits light by virtue of recombination energy of holes injected from an anode and electrons injected from a cathode by an application of an electric field. Since an organic EL device of the laminate type capable of being driven under low electric voltage has been reported by C. W. Tang et al. of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Page 913, 1987, or the like), many studies have been conducted for an organic EL device using an organic material as a constituent material. Tang et al. used tris(8-quinolinolato)aluminum for a light emitting layer and a triphenyldiamine derivative for a hole transporting layer. Advantages of the laminate structure include the followings: an efficiency of the hole injection into the light emitting layer can be increased; an efficiency of forming excitons which are formed by blocking and recombining electrons injected from the cathode can be increased; and excitons formed within the light emitting layer can be enclosed. As described above, for the device structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer, an electron transporting (injecting) layer, and the like are widely known. In order to increase the efficiency of recombination of injected holes and electrons in such devices of the laminate type, the device structure and the process of forming the device have been studied.
In general, when an organic EL device is driven or stored in an environment of high temperature, there occur adverse affects such as a change in luminescent color, a decrease in emission efficiency, an increase in driving voltage, and a decrease in a lifetime of light emission. In order to prevent the adverse affects, it has been necessary that the glass transition temperature (Tg) of the hole transporting material be elevated. Therefore, it is necessary that many aromatic groups be held within a molecule of the hole transporting material (for example, an aromatic diamine derivative of Patent Literature 1 and an aromatic fused ring diamine derivative of Patent Literature 2), and in general, a structure having 8 to 12 benzene rings is preferably used.
However, in the case of a highly symmetrical compound and a compound high in flatness each having a large number of aromatic groups in a molecule, crystallization is liable to occur upon fabrication of the organic EL device through the formation of a thin film by using those hole transporting materials. As a result, there arises a problem such as clogging of an outlet of a crucible to be used in vapor deposition or a reduction in yields of the organic EL device due to generation of defects of the thin film resulting from the crystallization. In addition, a compound having a large number of aromatic groups in any one of its molecules generally has a high glass transition temperature (Tg), but has a high sublimation temperature. Accordingly, there arises a problem in that the lifetime of the compound is short probably because a phenomenon such as decomposition at the time of the vapor deposition or the formation of a nonuniform deposition film occurs.
Meanwhile, Patent Literature 3 reports such an amine compound that heterocycles linked to each other are bonded to an amine, but the compound does not show sufficient performance when used in an organic EL device.
In addition, Patent Literatures 4 and 5 each report such an amine compound that heterocycles directly linked to each other are bonded to an amine through an aryl group, but the compound does not show sufficient performance when used in an organic EL device. In addition, Patent Literature 5 merely gives examples of such compound, and describes neither an example in which the synthesis of the compound is performed nor an example in which the compound is used in an organic EL device. In addition, Patent Literature 6 reports such a diamine compound that two amines are each bonded through an aryl group to a substituent to which a heterocycle is directly linked, but the compound does not show sufficient performance when used in an organic EL device. In addition, Patent Literature 7 reports such a diamine compound that two amines are directly bonded to a substituent to which two heterocycles are each linked through an aryl group, but the compound does not show sufficient performance when used in an organic EL device. Patent Literature 8 reports such an amine compound that heterocycles linked to each other through an aryl group are bonded to an amine through an aryl group, but the amine compound does not show sufficient performance when used in an organic EL device.
As described above, high-efficiency, long-lifetime organic EL devices have been reported, but none of them provides sufficient performance, and hence the development of an organic EL device having additionally excellent performance has been strongly desired.